In the FTIR, an infrared interference wave whose amplitude oscillates is generated by an interferometer, such as a Michelson interferometer, and is cast to a sample. Light transmitted by the sample or light reflected by the sample is detected as an interferogram, and the interferogram is subjected to Fourier transformation to obtain a spectrum (power spectrum) with the wavenumber as the abscissa and the intensity as the ordinate. Here, the Michelson interferometer is a device including a beam splitter (half mirror), a fixed mirror, a moving mirror. Light is divided into two light beams by the beam splitter, one of the light beams is reflected by the fixed mirror while the other light beam is reflected by the moving mirror, and then these two reflected light beams are made to interfere with each other. The amplitude of the resulting interference light oscillates with time as a result of moving the moving mirror.
The power spectrum obtained from light transmitted or reflected by a sample as described above is referred to as “sample spectrum” in the present description. Since a sample spectrum contains a background, it is necessary to subtract the background from the sample spectrum in order to obtain an absorption spectrum or a transmission spectrum. For that subtraction, conventionally, in the FTIR, a measurement is performed without sample before sample spectrum measurement to obtain a spectrum of the background.
If the direction of the moving mirror changes (if the moving mirror pitches or yaws) while a sample spectrum and a background spectrum are measured, temporal fluctuations may occur in the amplitude of the resulting infrared interference wave. Therefore, in the FTIR, in order to prevent fluctuations in the amplitude of the infrared interference wave due to change in the direction of the moving mirror, dynamic alignment is widely used (Patent Literature 1).
In dynamic alignment, laser light with a cross-sectional area smaller than that of infrared light to be used to obtain an interferogram in a main interferometer (Michelson interferometer) is used to perform a preliminary measurement for measuring change in the direction of the moving mirror while in motion. The preliminary measurement is made by detecting, with a photodiode array, movement of the position of a spot of the laser light during movement of the moving mirror with no sample provided. Based on a result of the preliminary measurement, when an FTIR measurement is performed, the direction of the fixed mirror is continuously adjusted during movement of the moving mirror so as to cancel out the effect of the change in the direction of the moving mirror. The adjustment of the direction of the fixed mirror is made by, for example, applying a voltage to a piezoelectric element provided on the back of the fixed mirror.